Alkynes reduction, stereospecific

Trans- and c -alkenes are made by stereospecific reductions of corresponding alkynes. 

Alkenes can also be produced effectively by die reduction of alkynes. The reduction can be carried out stereospecifically to give either cis or trails olefins as desired. This is a very useful method because of the stereocontrol. The P-2 nickel catalyst for the cis hydrogenation is produced in situ by the reduction... 

Alkene synthesis is the most important and most widely used reaction in organic chemistry. Many review articles and research papers describe methods for the positioning and stereospecific introduction of carbon-carbon double bonds ". The three most important methods for the synthesis of alkenes described in this chapter are (1) elimination reactions, (2) aUcenation of carbonyl compounds and (3) reduction of alkynes. 

Alkynes are reduced to trarts-alkenes with Na or Li and liquid NH3 that contains stoichiometric amount of alcohol. This reduction, known as Birch reduction , is highly selective as no saturated product is formed and completely stereospecific as only trans-alkene is formed. 

The partial reduction of alkynes provides methods that are both regio-and stereospecific. Dissolving metal reductions tend to give trans alkenes, whereas catalytic methods of reduction generate the cis alkenes (Scheme 3.3). A Lindlar catalyst (Pd/CaCOj + PbO, partially poisoned with quinoline) has been recommended for use in this context. 

The stereoselectivity in equations (90) and (91) indicates, albeit in oversimplified form, the possible difference in syn and anti reductions. Process (90) is stereospecific in THF—only the F-alkene is produced in toluene, both alkenes and the alkane are produced (see Table 11) . Process (91) is highly selective, yielding 98% of the -alkene in THF but yielding some of the Z-alkene, i.e. EjZ = 3/1, in ether . Our interpretation of these results is that in the more polar solvent, THF, in which LAH is probably somewhat dissociated , normal ionic addition occurs a coordinating metal ion (if any) and a final proton come in anti from the medium—hence the ionic representation. In the less polar solvents, ether and toluene, H and then metal (M) are delivered syn from associated LAH to one side of the alkyne—hence the aggregate representation. 

Trans- and CK-alkenes are made by. stereospecific reductions of corre.sponding alkynes. 

The ease of reduction is 1-alkynes > disubstituted alkynes > 1-alkenes, indicating that selective semihydrogenation of triple bonds can be achieved in molecules containing double bonds. For example, nonconjugated enynes can be hydrogenated regio-and stereospecifically in high yields. 

Dear and Pattison regard lithium-ammonia as superior to sodium-ammonia for the stereospecific /rans-reduction of alkynes. The reaction is carried out in tetrahydrofurane under pressure in an autoclave at room temperature. 

However, obtaining the thermodynamically less stable c -isomer is not so clear cut. Often the rran.r-isomer is present in lesser amounts than the m-isomer. The reduction of NiBr2 2DME by K graphite affords dispersed Ni on the graphite surface (Ni—Gr—1). Freshly prepared Ni—Gr—1 is used in situ as a semihydrogenation catalyst for alkynes with a stereospecificity of 93-99% ... 

Palladium-catalyzed hydrostannation of alkynes proceeds regio- and stereospecifically to afford the synthetically useful ( )-vinylstannanes. This reaction implies oxidative addition of RsSn—H to Pd(0) to generate a Pd(ll) hydrido stannyl intermediate, which then undergoes cis addition of the Pd—Sn bond to the alkyne bond, followed by reductive elimination of the ( )-vinylstannane. The supposed cis-PdCII) hydrido trialkylstannyl intermediates had so far remained elusive. Very recently, cis-PdCll) hydrido trialkylstannyl complexes have been synthesized for the first time. ... 

The conversion of an unsaturated acid or ester to its perhydro derivative-sometimes a significant step in the identification of new fatty acids-is readily achieved by catalytic hydrogenation using a platinum, palladium, or nickel catalyst. The partial and stereospecific reduction of alkynes to cis alkenes is most commonly effected with Lindlar s catalyst (Section 7.1). 

Again, as seen with alkenes, both borane (B2H6) (Equation 6.22) and diimide (H-N=N-H) (Scheme 6.15) can be used to reduce alkynes. Indeed, the reaction of internal alkynes with borane is apparently more facile than that with the alkene that results from consummation of the reduction. Further, as would be expected, suprafacial addition of hydrogen and boron obtains and Z- (or cis-) alkene is the only product. The use of deuterated boron compounds (e.g., the hindered 9- H-9-borabicyclo[3.3.1]nonane [9- H-9-BBN]), commercially viable since it contains only one deuterium ( H), followed by use of deuterated acetic acid (CH3C02, i.e., reductive workup) produces Z- (or cw-)-dideuteroalkene of high stereospecificity and in high yield (Scheme 6.65). 

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